1. Field of the Invention
The present invention relates to a composite insulator including a hollow and tapered core portion, a sheath formed on the outer peripheral surface of the core portion and a plurality of sheds which are formed on the sheath and axially spaced from each other. The present invention also relates to a method and apparatus for manufacturing such composite insulators.
2. Description of the Related Art
In general, a hollow composite insulator includes a hollow core portion consisting of fiber reinforced plastic (FRP), having an outer peripheral surface which is covered by a sheath over substantially the entire length thereof. A plurality of sheds are provided on the sheath and axially spaced from each other. The sheath and the sheds are composed of an insulating polymeric material. This type of hollow composite insulator has been actually used in practical applications, particularly in any use environment which can exploit the various functional advantages of the composite material. In this instance, silicone rubber, ethylene propylene copolymer (EPM), ethylene propylene diene terpolymer (EPDM), etc., are used as the polymeric materials for the sheath and the sheds.
Such a composite insulator is disclosed, e.g., in FR-A-2363170 wherein the insulator includes a hollow and tapered core portion which is comprised of fiber-reinforced plastics. A sheath comprised of an insulating polymeric material is integrally adhered to an outer peripheral surface of the core portion by vulcanization, for example. A band also comprised of an insulating polymeric material and having a substantially T-shaped cross section is helically wound around the sheath, so that the head of the letter T is adhered to the sheath and the legs of the letter T form the sheds which are axially spaced from each other in the longitudinal section of the insulator. In this instance, the band has two edge portions in the form of slant surfaces parallel to each other, and is wound around the sheath such that opposite edge portions of adjacent turns of the band are brought into tight contact with each other. Such an arrangement of the composite insulator makes it difficult to simplify and facilitate manufacturing operation and to improve the dimensional accuracy of the product, because the sheath and the sheds must be formed in different processes, and the band must be wound on the frustoconical outer peripheral surface of the core portion of a hollow and tapered structure while strictly maintaining a geometrical relationship whereby the opposite edge portions of the adjacent turns of the band are in a tight contact with each other.